1. Field of the Invention
The present invention generally relate to a navigation satellite system (NSS) receiver, and more specifically to a differential Teager-Kaiser-based code tracking loop discriminator for multi-path mitigation used in the receiver.
2. Description of the Related Art
One popular system for terrestrial location and velocity estimation, known in the art as the global positioning system (GPS), comprises a set of orbiting satellites, ground control stations, and user devices known generally as GPS receivers. A GPS receiver conventionally includes an antenna, a radio frequency front end, down conversion circuitry, an analog to digital converter subsystem, and a digital signal processing subsystem. The GPS receiver computes a location based on a set of signals received from a corresponding set of satellites, each with a known orbital pattern. To accurately compute a location, the GPS receiver needs to receive a satellite signal from a minimum set of typically four satellites. Each satellite signal includes position (“ephemeris”) and time information modulated by a wide band pseudorandom noise (PRN) code. A PRN code comprises a sequence of “chips,” wherein the sequence or a sub-sequence thereof forms a code signifying one bit of data. Different PRN codes and bit rates may be used for different purposes. For example, one PRN code is referred to in the art as a coarse acquisition (CA) code, and comprises 1023 chips. The chips are transmitted at a rate of 1.023 MHz.
Each satellite signal typically arrives at the GPS receiver below a typical receiver noise floor. However, the GPS receiver knows each PRN code and is able to correlate the satellite signal with known PRN code, thereby imparting processing gain on the data and timing information encoded by the transmitted PRN code. With sufficient processing gain, the satellite signals may be recovered by the GPS receiver.
Each satellite signal ideally follows a direct line-of-sight (LOS) path from a corresponding satellite, through the atmosphere, to the GPS receiver. However, one or more of the satellite signals may actually follow multiple paths as a result of reflecting off buildings or the ground before arriving at the GPS receiver, thereby giving rise to multipath errors. Multipath errors directly impact code tracking performance, which leads directly to measurement errors. Multipath errors are commonly measured relative to chip time.
Multipath mitigation for GPS falls into two primary categories, including receiver-external approaches and receiver-internal approaches. Receiver external approaches use techniques that are independent of the receiver hardware or software, such as using antenna design and antenna placement. Receiver internal approaches may include measurement processing (navigation) level or signal processing level techniques. Measurement processing approaches use calibration and repetitive sampling using signals from multiple satellites to estimate corrections for current and future measurements. Signal processing approaches conventionally use known characteristics of LOS and multipath signals at link level (i.e. taking into account the signal from one satellite at a time) to attenuate effects of the multipath signal. One conventional signal processing approach uses multiple correlators to estimate parameters of the LOS and multipath signals to track and mitigate multipath errors.
A second approach may track the LOS signal by separating the LOS signal from multipath replicas. This technique uses a multi-correlator sampling of autocorrelation functions and may also use discriminator curves. Additional correlators beyond early, late, and prompt correlators are often required along with wider bandwidth in certain system elements. This second approach has varying effectiveness depending on multipath signal characteristics. For example, as a multipath parameter is closer to a LOS parameter, the less effective this approach is in distinguishing the multipath signal from the LOS signal. Thus, tracking only the LOS signal tends to be more effective in scenarios with long multipath delays and a high signal to noise ratio (SNR). However, users frequently encounter usage scenarios with shorter multipath delays and lower SNRs, which can significantly degrade performance and accuracy of this second approach.
Therefore, a technique for more robust multipath mitigation in a navigation satellite system (NSS) receiver is needed.